1. Field of the Invention
The present invention relates to a light-emitting diode, and particularly, to a light-emitting diode capable of conducting large electric current with high efficiency.
2. Description of the Related Art
In recent years, attempts have been positively made to apply light-emitting diodes to illumination because red to blue colors have been completed, and light-emitting efficiency of light-emitting diodes can finally be obtained to be equal to light-emitting efficiency of bulbs.
On the other hand, the conducting current of utility light-emitting diodes is generally 20 mA, but one light-emitting diode only serves as a light-emitting source on the order of a few tens of mW. To obtain brightness of the order of a few tens of W as in bulbs, plural light-emitting diodes are connected in parallel or series to each other, to obtain brightness needed. In traffic lights, for example, about 200 light-emitting diodes arranged on the plane for one bulb are used as a lamp. Accordingly, to widely use light-emitting diodes for illumination, it is required that light-emitting efficiency is higher, and that conducting large current is possible, rather than that energy consumption and cost are reduced.
Conventional light-emitting diodes are very easily affected by heat in comparison to conventional lamps such as bulbs, fluorescent lamps, etc. Heat caused during large electrical conduction through the light-emitting diodes degrades light-emitting efficiency and reliability. To avoid this, there are methods for rapidly dissipating heat caused to a stem, or for preventing generation of heat as much as possible.
The method for rapidly dissipating heat caused is as follows: Flip-chip structure is formed as in FIG. 9, which comprises a light-permeable substrate 109 having an extracting surface on its front side; a light-emitting layer having a p-type cladding layer 102, an active layer 103, and an n-type cladding layer 104 on the back side of the substrate 109; a packaging stem 107; an electrode 110 for the n-type cladding layer 104; an electrode 111 for the p-type cladding layer 102; and a packaging alloy 112, wherein current is injected from the electrode 110 for the n-type cladding layer 104 and the electrode 111 for the p-type cladding layer 102 bonded to the packaging stem 107 via the packaging alloy 112, and wherein the light-emitting layer that is a heat-generating portion is positioned as close to the packaging stem 107 as possible. However, in this flip-chip structure, although heat caused in the light-emitting layer can be dissipated efficiently to the packaging stem 107, there is the problem with thermal resistance between the heat-generating light-emitting layer and the heat-dissipating substrate.
Accordingly, as shown in FIG. 10, a light-emitting layer comprising an n-type cladding layer 104, an active layer 103, and a p-type cladding layer 102 is first grown over a high thermal-resistive substrate (not shown), and laminated with a low-thermal-resistive high-thermal-conductive substrate 113 via a semiconductor bonding layer 114, followed by removal of the high thermal-resistive substrate, and subsequent formation of upper and lower electrodes 101 and 106. A Si substrate is most widely used as the high-thermal-conductive substrate 113 used in FIG. 10. Also used is a substrate which uses CuW, etc. from a linear expansion coefficient relationship.
On the other hand, to prevent generation of heat as much as possible, light-emitting efficiency of light-emitting diodes has to be made high, and electrical energy converted into light as much as possible for extraction, so as not to be converted into heat. In other words, it is required that internal quantum efficiency for efficiently recombining electrons and holes injected is made as high as possible, and further, that light-extracting efficiency for extracting light emitted from light-emitting diodes is made high.
In the light-emitting diode of FIG. 10, however, of light emitted from the light-emitting layer to the front surface, a portion of the light is released to outside of the light-emitting diode, but from a difference between the refractive index of the surface of the light-emitting diode and the refractive index of the outside of the light-emitting diode, most of the light is reflected at the surface of the light-emitting diode, so that light passed towards the high-thermal-conductive substrate 113 reaches the bonding layer 114 at the interface of the light-emitting layer and the high-thermal-conductive substrate 113, where some of the light is reflected and the other is absorbed. If the light reflectance at the bonding layer 114 is high, light reflected is passed to the front surface, and a portion of the light is released to the outside of the device, while the remaining light is again reflected. Light can be extracted by repeating such reflection. However, because it is also required that the bonding layer 114 conducts electricity, its reflectance cannot be made very high. In other words, there is a tradeoff between light reflectance and electrical conductance.
To circumvent this, as shown in FIG. 11, there is the method of separately forming the current-conducting partial electrode 115 and the light-reflecting bonding layer 114 in the light-emitting diode of FIG. 10 (See JP-A-2001-144322, for example). Further, directly below the light-impermeable upper electrode 101 is formed a current-blocking portion 116, to spread current to the periphery of the upper electrode 101 and thereby prevent light from being emitted directly below the upper electrode 101, to enhance light-emitting efficiency.
In the structure of FIG. 11, however, while the driving voltage can be controlled to be low by increasing the current-conducting area of the partial electrode 115, because the area of the bonding layer 114 is decreased, there is the problem of a decrease in reflectance and therefore in light-emitting efficiency. In other words, in the structure of FIG. 11, there is difficulty in balancing the tradeoff between high reflectance and low electrical resistance.